This invention relates to a method of and an apparatus for controlling the brake torque of an eddy current brake by varying the energizing voltage in the energizing coil of the eddy current brake.
Eddy current brakes are used for testing motors of all kinds. These brakes take up the energy output of the motor being tested by converting such energy into heat. The heat is then discharged through a cooling medium. The energy output of the motor being tested is usually determined by measuring the torque and the rotational speed or r.p.m. of the motor. The torque absorbed by the eddy current brake is determined by the magnitude of the direct current which flows through the energizing coil of the eddy current brake. The torque absorbed by the eddy current brake is usually referred to as the "brake torque".
However, during the dynamic operation of the brake a time-lag variation occurs in the energizing current as the energizing voltage changes in time. This time-lag is a constant. The respective time constant is determined by the coil and is directly proportional to the self induction of the coil and inversely proportional to the ohmic resistance of the energizing coil.
For testing the dynamic performance of a motor, it is necessary to change the torque absorbed by the brake as quickly as possible. To achieve such rapid torque changes, it is necessary to reduce the rise time of the energizing current which may be accomplished by increasing the ohmic resistance in the energizing coil circuit, whereby the time constant becomes smaller. However, the energizing voltage must be considerably increased during the entire time that the brake is in operation. The operational costs of the brake are thereby increased since the excess voltage cannot normally be utilized effectively.
Another method that has been used is to control the energizing current directly. Both control methods are successfully applied to direct current machines. However, it has been found that in this manner it is not possible to decrease the energizing or de-energizing times of eddy current brakes to any desired extent.